SUMMARY Rabbit transplantation sera were studied for their cytotoxic activity against human lymphocytes. Of 41 skin graft sera tested, 15 gave definite positive reactions with some lymphocyte samples. The family studies showed that the human lymphocyte isoantigens recognized by rabbit transplantation sera are inheritable, and they appear to be controlled by a single chromosomal locus which seems to be closely linked or identical with the HL-A locus